Genes encoding growth factors and their receptors have been implicated in the regulation of normal cell growth and development. There is also increasing evidence that genetic alterations affecting expression of such genes can contribute to altered cell growth associated with malignancy. The normal homologues of some oncogenes code for membrane-spanning growth factor receptors with tyrosine kinase activity (2, 3). Other oncogenes appear to act in pathways of growth factor activated cell proliferation as well (4). Thus, increased knowledge of growth factor regulatory systems in general is expected to provide better understanding of genes critically involved in both normal growth control and neoplasia.
Platelet-Derived Growth Factor (PDGF) is of particular importance because it is a major connective tissue cell mitogen which is thought to play a major role in normal wound healing. Further, the abnormal expression of PDGF has been implicated not only in cancers, but also in a variety of histopathologic states including arteriosclerosis, arthritis, and fibrotic diseases (23).
PDGF consists of a disulfide-linked dimer of two polypeptide chains, designated A and B. There is evidence for the natural occurrence of all three possible dimeric structures containing A or B chains or both (1, 25, 26). The various dimeric forms of the growth factor are called "isoforms". A variety of normal and neoplastic cells appear to specifically express either the A or B chains. Nevertheless, the most significant human isoform for physiological regulatory processes is believed to be the one isolated from human platelets, namely the AB heterodimer (i.e., a dimer containing one A and one B chain; see reference 24).
The PDGF-A and B chains have distinguishable properties (37). The A chain is much more efficiently secreted and exhibits lower specific mitogenic activity than the B chain. The B chain gene of PDGF has been shown to be the normal human homologue of the simian sarcoma virus-derived v-sis oncogene. Moreover, there is accumulating evidence that expression of the B chain in cell types possessing PDGF receptors can drive such cells along the pathway to malignancy. The A chain is less potent than the B chain in inducing neoplastic transformation of cultured mouse (NIH/3T3) cells.
Recent studies have suggested the existence of two subtypes of the PDGF receptor (PDGF-R), on the basis of PDGF isoform binding and competition using mouse or human fibroblasts (27). These works are consistent with the hypothesis that there exists one receptor subtype which preferentially binds the B chain dimer, and another which efficiently binds all isoforms of the PDGF molecule. However, the results of these studies could not discriminate between two distinct possibilities with differing implications for the study and ultimate treatment of diseases involving such receptors: either these subtypes represent differently processed products of a single PDGF-R gene; or they are products of distinct genes.
Further, there have been conflicting findings concerning binding of different PDGF isoforms of the receptor produced by a previously identified human PDGF-R gene. Introduction of PDGF-R genes by expression vectors into different cell types devoid of PDGF receptors has been reported to lead either to preferential binding of PDGF-BB (14) or, alternatively, to efficient binding by all three isoforms (28). The basis of this discrepancy is not known.
Thus, there has been uncertainty concerning the ability of the known PDGF receptor to respond to different PDGF isoforms, and to the main AB heterodimer form of human PDGF, in particular. Some reported differences might be explained by cell specific differences in post-translational processing of the product of the known PDGF-R gene, or by the presence of accessory proteins in certain cell types. Alternatively, the different binding properties reported in different studies might be explained by the existence of two distinct genes encoding different PDGF receptors.
In light of the complexities of PDGF ligand and receptor activities described above, and the related processes which are influenced thereby, comprising both normal wound healing and abnormal connective tissue conditions, including neoplastic growth, arteriosclerosis, arthritis, and fibrotic diseases, it is apparent that there has been a need for methods and compositions and bioassays which would provide an improved knowledge and analysis of mechanisms of connective tissue growth regulation, and, ultimately, a need for novel diagnostics and therapies based on the PDGF receptors involved therein.
In particular, the observations above, indicate a specific need for thorough characterization of the genetic basis of PDGF receptor production. Furthermore, it has been shown previously (5) that it is possible to identify and clone novel related members of the gene family encoding membrane-spanning growth factor receptors with tyrosine kinase activity, which comprises the known PDGF receptor gene and the kit and fms oncogenes, by exploiting the conserved tyrosine kinase coding region as a probe.
Accordingly, the present invention contemplates the application of methods of recombinant DNA technology to fulfill the above needs and to develop means for producing PDGF receptor proteins which appear to be the predominant effectors of the main form of human PDGF. This invention also contemplates the application of the molecular mechanisms of these receptors related to healing and pathological processes.
In particular, it is an object of the present invention to identify and isolate the coding sequence of a novel human gene related to but distinct from the known PDGF-R gene, as well as from other members of the family of tyrosine kinase genes comprising the PDGF-R, kit, and fms genes. Further, it is an object of this invention to develop the molecular tools needed to establish the relative roles of the novel and known forms of PDGF receptor in physiological processes involving PDGF.